[unreadable] Exposure to a wide range of environmental agents can cause dramatic changes in gene expression. In theory, these changes should revert back to normal after exposure has ceased. It appears, however, that changes in gene expression can persist after acute or chronic exposures suggesting epigenetic changes, and it has been suggested that these changes can have pathological consequences. For example, silencing of critical genes is commonly observed in environmentally induced cancers. The goal of the work proposed in this exploratory R21 application is to test the hypothesis that reductions in gene expression occurring in response to environmental exposures will sensitize mammalian promoters to undergo silencing. A corollary of this hypothesis is that selective pressure, which propels the development and progression of cancerous cells, can also propel the silencing process. Three specific aims are offered. For the first aim, expression of the selectable mouse Aprt (adenosine phosphoribosyl transferase) coding region will be controlled with the tet on/off system in cultured mouse cells and the relation between chronic or acute reductions in expression and stable silencing will be examined. The role of a pre-existing focus of DNA methylation in facilitating the silencing process will also be examined. For the second specific aim, Aprt will be expressed from nickel (Ni) repressible promoters and the ability of this common environmental carcinogen to trigger stable silencing by repressing transcription will be examined in mouse cells. For the third specific aim, the ability of a variety of environmental carcinogens to trigger silencing via promoter repression will be examined in the MCF-7 human breast cancer cell line. The work for this aim will use Hprt as the selectable marker for silencing. Successful completion of the proposed work will firmly link environmental exposure with gene silencing and demonstrate that one mechanism for this link is a significant reduction in promoter function. [unreadable] [unreadable] [unreadable] [unreadable]